The invention is directed to an improvement in filtering hot, unsaturated, saturated or supersaturated slurry, using a rotary vacuum precoat filter, especially in filtering a phosphoric acid slurry formed during the manufacture of phosphoric acid and containing typically up to about 15%, more usual 3-6%, by weight of solids.
The invention involves a process for removing solid impurities from impure phosphoric acid containing solids and comprises:
passing said acid at an elevated temperature through a filter cake of a phosphoric acid-resistant filter aid supported on a porous medium, thereby to cause at least a portion of said solid impurities of said acid to be filtered therefrom and to be retained by said filter cake; PA1 contacting at least a portion of said filter cake with a displacing liquid, maintained at a temperature and in an amount sufficient to cause at least a portion of the acid which remains in said filter cake to pass through said filter cake without fracturing said filter cake; PA1 collecting said filtered acid which passes through said filter cake; and PA1 passing said acid at an elevated temperature through a filter cake supported on a porous medium, thereby to cause at least a portion of said solid impurities in said acid to be filtered therefrom and to be retained by said filter cake along with some of the acid; PA1 contacting at least a portion of said filter cake with a displacing fluid, preferably a liquid, maintained at a temperature and in an amount sufficient to cause at least a portion of the acid retained in said filter cake to pass through said filter cake without fracturing said filter cake; PA1 collecting said filtered acid which passes through said filter cake; and
removing an impurity-containing portion of said filter cake after contact with said acid and displacing liquid, thereby to present a fresh surface for subsequent filtration.
The amount of the displacing fluid, its pressure, and the angle of the spray relative to the filter cake are preferably chosen so as to minimize penetration of the solid impurities from the surface of the filter and cake into the interior of the cake.
Phosphate rock is mined principally for conversion to phosphoric acid and thereafter to high phosphate fertilizer products. Mined phosphate rock, usually after beneficiation (e.g. clay and sand removal), is solubilized with phosphoric acid and reacted with sulfuric acid to produce phosphoric acid solution and insoluble calcium sulfate. Such a process is generally referred to in the industry as the wet process of phosphoric acid production. After separation from the bulk of the solid calcium sulfate, usually by filtration, the resultant crude phosphoric acid solution is further processed to produce fertilizer products.
The crude phosphoric acid includes substantial proportions of impurities, such as compounds of fluorine; calcium, determined as CaO; sodium, expressed analytically as Na.sub.2 O; silicates, expressed analytically as SiO.sub.2 ; SO.sub.4 ; iron, expressed analytically as Fe.sub.2 O.sub.3 ; aluminum, expressed analytically as Al.sub.2 O.sub.3 ; and magnesium, expressed analytically as MgO.
The relative proportions of impurities contained in the crude phosphoric acid are largely determined by the type of wet processing utilized to produce the crude phosphoric acid and the nature of the phosphate-bearing rock from which the crude phosphoric acid is produced. Sufficient proportions of the impurities remain in the acid even after initial aging and settling and after a substantial period of time, relatively fine solids precipitate and separate from the acid in the form of a sludge. It is usually commercially impractical to hold the crude phosphoric acid sufficiently long enough to allow the formation and separation of the sludge.
It has been found, as disclosed in the U.S. patent application Ser. No. 827,741, filed Aug. 25, 1977, now U.S. Pat. No. 4,235,584 of Smith, et al, that by preheating the acid, followed by vacuum filtration through a filter medium including a diatomite cake, sludge forming impurities can be removed in sufficient quantity to substantially eliminate the subsequent formation of sludge during shipping and handling of the phosphoric acid product.
Sludge forming impurities can be removed from phosphoric acid by the processes disclosed in U.S. Pat. No. 4,136,199, and Ser. No. 954,647, the entire disclosures of which are incorporated herein by this reference.
Preferably, the filtration of the crude phosphoric acid is conducted using a rotary vacuum precoat filter. Rotary vacuum precoat filters are well-known in the art.
U.S. Pat. No. 3,907,680 discloses a process for purifying a crude aqueous wet process produced phosphoric solution which includes passing the solution through a layer or bed of perlite (an alumino-silicate). The patent further teaches that this purification technique may be accomplished via a vacuum filter. The patent does not disclose the use of diatomaceous earth as the filter precoat or the use of a water spray to remove entrained phosphoric acid from the filter cake.
A Johns-Manville brochure discloses a rotary vacuum filter containing a (Celite) filter aid precoat. Celite is Johns-Manville's trademark for its diatomaceous silica products. Among the numerous applications listed in the back of the brochure for Celite filter aids is the treatment of phosphoric acids. The rotary vacuum precoat filter device pictured on page 17 of the brochure includes means for applying a wash liquid to the filter cake. However, the brochure does not disclose maintaining the temperature of the wash liquid at the same temperature as the filter cake to remove entrained phosphoric acid without fracturing the filter cake, nor is any distinction made between washing a desired solid product and selective displacement from an undesired solid filter cake of a desired liquid.
U.S. Pat. No. 3,200,105 discloses a method for removing catalyst from a polymer product by passing the product through a precoated filter. An organic solvent wash is used following filtration to remove entrained product from the filter precoat. However, the patent contains no disclosure of temperature control of the organic solvent.
Other patents of general relevance in teaching the use of diatomaceous precoats on rotary filters are U.S. Pat. Nos. 3,616,908, 3,520,410, 3,502,575, 3,080,971 and 2,403,021. Processes in which phosphoric acid is filtered to remove solids are found in U.S. Pat. Nos. 4,164,550, 4,110,420 and 3,528,771.
With the use of rotary vacuum precoat filters, as well as other types of precoat filters, some P.sub.2 O.sub.5 values can be lost in the precoating of filter aid. That is, the precoat cake can entrap and retain P.sub.2 O.sub.5 values in the interstices of the cake. It would be desirable to develop apparatus and a process for recovering such P.sub.2 O.sub.5 values.
By phosphoric acid resistant filter aid, we mean that the filter aid does not significantly lose its ability to aid in filtering fine particles from phosphoric acid, although in many cases, the filter aid can be partially digested upon exposure to the phosphoric acid being filtered.